The Witcher 2 mutagens

In The Witcher 2: Assassins of Kings, mutagens are powerful mutation-triggering agents and (with patch 1.3 installed) alchemy ingredients. They allow you to raise your abilities even further. Mutagens can be acquired in two ways: you can loot them from defeated enemy corpses or you can create them as random by-products of alchemy. To undergo mutation, enter meditation and then select the "Character" option and find a skill with a mutagen slot. Mutations cannot be reversed; as such, it is strongly recommended that you never use lesser or basic mutagens, only greater ones. It is also recommended to have the Impregnation talent from the alchemy tree at level 2 before using any mutagens, because that skill does not affect mutagens which have already been applied.

(For those not focusing on the alchemy tree) As of patch 1.1, it is possible to enjoy the Impregnation benefit without ultimately keeping the skill. One may put enough points in the alchemy tree to get Impregnation to level 2, then invest in other trees (not the alchemy tree) as much as possible to get Mutagen skills. In Chapter 3, before you meet The Operator during the quest From a Bygone Era, make sure you have all the mutagens applied already. Then accept his offer to reallocate your talents (using the mysterious artifact). In the immediate talent distribution screen, re-pick all the skills in trees except the alchemy tree. The improved mutagens will remain even though you no longer have Impregnation.

(as of patch 2.1) The effects of mutagens used remains even without selecting the skils. After accepting the offer to re-allocate skill points, you will notice that attributes are boosted even without selecting any skills. It is possible to utilise all 13 slots this way. It however resets back to normal once you save and reload.

If you have seen a frustratingly low number of greater mutagen drops from slain enemies, you might try allocating some points into Side Effect. With 2 points in this alchemy tree skill there will be a 10% chance of mutagens being created as a byproduct of alchemy. Often multiple mutagens are created simultaneously from the same alchemical concoction. There are some alchemical concoctions that require only a few common ingredients, such as the Grapeshot bomb. You can create them one at a time, and they occasionally produce several greater mutagens in the same go. If you do not want to spend the points in that skill, you could do it early in the game to harvest mutagens, then use the impregnation exploit described above.